Written by Merel Dagher (PEACE Advisor)
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The correct answer is b. Cyclin B is needed, with CDK, to propel the cells from G2 to M phase. Cyclin A is needed, with CDK, to propel the cells from G1 to S phase. Cyclin A and B are constantly produced, however, depending on which part of the cell cycle it is, they are tagged by ubiquitin and degraded by the proteasome.